Digital fine-coarse servomechanism for a single element printer control system

ABSTRACT

A single element printer selection servomechanism for selectively positioning a spherical printing head to one of a plurality of printing positions arrayed about the printing element. Each printing position comprises a type character disposed within a sector on the surface of the printing element. An error signal representative of the difference between actual and commanded position of the printing element energizes a motor to roughly position the sector of the printing element containing the selected type character. Fine positioning means provide a signal to precisely position the midpoint of the sector of a selected type character after the error signal has been reduced to zero and rough positioning taken place. Circuit means are also provided for braking the motor when its speed reaches a predetermined amount relative to the amplitude of the error signal.

United States Patent Crosby 51 June 13, 1972 [72] Inventor: Donald P.Crosby, Ridgefield, Conn.

[73] Assignee: Sperry Rand Corporation, New York,

[22] Filed: Dec. 12, I969 [21] Appl.No.: 884,744

[56] References Cited UNITED STATES PATENTS 2,907,937 10/l959 Apgar eta1. ..3l8/594 3,370,289 2/1968 Hedgcock et al. .318/594 X 3,372,3213/1968 lhaba et a1. ....318/594 3,378,741 4/1968 Sutton ..3 1 8/5943,473,009 10/1969 Gerber et a1. ..318/594 X 2,885,613 5/1959 Myrache etal.... .....318/20.330 3,369,160 2/1968 Kappel et al ..3 l 8/20.54$ X3,399,753 9/1968 Revelle ..3 18120.3 1 5 X Primary Examiner-T. E. LynchAttorney-Marshall M. Truex, Frank A. Seemar, H. Walter Clum and ThomasP. Murphy ABSTRACT A single element printer selection servomechanism forselectively positioning a spherical printing head to one of a pluralityof printing positions arrayed about the printing element. Each printingposition comprises a type character disposed within a sector on thesurface of the printing element. An error signal representative of thedifference between actual and commanded position of the printing elementenergizes a motor to roughly position the sector of the printing elementcontaining the selected type character. Fine positioning means provide asignal to precisely position the midpoint of the sector of a selectedtype character after the error signal has been reduced to zero and roughpositioning taken place. Circuit means are also provided for braking themotor when its speed reaches a predetermined amount relative to theamplitude of the error signal.

11 Claims, 6 Drawing Figures 22 4 [crew/c Pas/non a J 2:: 'f mam/AlvaV0046: MEMOEY 05:00:: 057mm 23 KAAIJ/Jrok Jw/rov s 2K f f A ,9 20 f/A/EPenna/wild muneoz P a la Z1;

firm/r Elf/VENT Pas/r10 1 6K1) f e c z (00! 0/52- arr/(At 5mm! Pmvup:00! (DA/Vikfik A/wz/F/m DIGITAL FINE-COARSE SERVOMECI-IANISM FOR ASINGLE ELEMENT PRINTER CONTROL SYSTEM This invention relates to a singleelement printer and more particularly to a servomechanism forselectively positioning the printing element of a single element printeror typewriter.

A spherical-shaped printing element is connected for rotational movementto a DC torque motor through a universal coupling. The torque motor isenergized with an analog error voltage representative of the position ofthe printing element relative to its commanded position. The position ofthe printing element is digitally encoded by means of a code wheelattached to the motor shaft for movement with the printing element. Anelectronic keyboard provides a discrete digital code for each characterkey depressed. Alternately the print character selection code may be theoutput of a computer or similar device. These digital codes areconverted to analog voltages and summed to produce the analog errorvoltage for driving the torque motor.

The printing element has 22 type characters in each of four rows. Eachtype character is equally spaced from adjacent type characters andoccupies a printing position within approximately a 16 sector on acircumference of the printing element. When a portion of a 16 sector ofa selected type character is positioned according to a command the errorsignal has become zero. Since it is necessary to precisely position thetype character which is at the midpoint of the sector, fine positioningmeans automatically take over when the analog error signal becomes zero.This fine positioning is also accomplished by means of the DC torquemotor 12 which receives positive or negative voltages provided inresponse to the least significant bits provided by the Gray to Binarycode converter I7.

The input to the torque motor during coarse positioning is changed inaccordance with the velocity of the motor. That is, when the motorsvelocity reaches a predetermined amount relative to the amplitude of theerror signal, a braking signal is applied to the motor.

The primary object of the present invention, therefore, is to provide asingle element printer selection system wherein an electronicallycontrolled servomechanism accurately positions the printing element of asingle element printer or typewriter within minimum time intervals.Since the character selection input to the present system is a digitalcode, it may serve as a print out or terminal element of an electronicdata processing system or the like.

Other objects of the present invention will become apparent with thereading of the following description wherein:

FIG. I is a block diagram illustrating the selection servomechanism ofthe present invention;

FIG. 2 is a representation of the printing element drive arrangement;

FIG. 3 is a flat view ofthe coded disc shown in FIG. 2;

FIG. 4 is a block diagram of the logic of the line positioning control;

FIG. 5 is a schematic representation of the power amplifier of thepresent invention and,

FIG. 6 is a diagram of the voltage inputs to the motor drive circuit.

Referring more particularly to FIGS. I and 2 there is shown a sphericalprinting element I 1 similar to the one used on the IBM Selectrictypewriter. As best seen in FIG. 2 the printing element may have fourrows of 22 type characters each. Each type character occupies themidpoint of a sector or are of somewhat more than 16. Thus, duringprinting impact the printing element II must present the midpoint of thel6 sector of the selected character.

FIG. 1 illustrates the block diagram of a system for accomplishing typecharacter selection with respect to a single row by rotation of theprinting element. The manner in which rows are selected by tilting theprinting element 11 is not a part of the present invention but will beexplained hereinafier to the extent necessary to fully explain thepresent invention.

A DC torque motor 12, which is of a type commercially available, isconnected to the printing element II through a universal coupling 13.The universal coupling 13 transmits the rotational drive from motor 12as well as permits printing movement of the printing element i.e., inthe direction perpendicular to the plane of FIG. 2.

Mounted for rotation with the output shaft of the torque motor 12 is acode disc 14. The code disc 14 is shown in FIG. 3 and has concentricslotted and non-slotted rings which are arranged to provide 44 discretecombinations of slots and noslots along as many radial sectors, Thus, anelongated source of light 15 so disposed as to project a line of lightextending alongaradiusofthecodedisc I4willproject throughthe disc 44different combinations of light-dark areas for each revolution of thecode disc 14.

The light code passing through the code disc I4 is converted toelectrical fon'n by means of an optical pick-up 16. The optical pick-up,a commercially available item, comprises six photo transistors (one foreach ring of code disc 14) arranged opposite from the light source onthe other side of the code disc 14. Thus, the optical pick-up I6 whichmay include an amplifier provides as an output a six bit digital codecapable of 64 variations. In the present case only 44 variations areused with each two being indicative of a segment on the disc I4. In thisway the instantaneous position of the printing element 11 has two codesfor each of its 22 type character printing positions. Since 64 codevariations are actually available a 32 position printing element ispossible without any additional circuitry.

The output via the code disc 14 is in the Gray Code. FIG. 3 shows theslot arrangement of the disc 14. Each radial line on the disc hm a codee.g., the dotted line shown has code O0l0l0. This and 00lll0 e.g., mayencode the position for the letter I: on the printing element since eachposition is represented by two discrete codes on the code disc 14. Theupper case of each letter is disposed away from the lower case. The codeof an upper case letter is found by adding I] bits to the code of thelower case number. The Gray Code is used for the actual positionencoding to diminish possibility of error since in the Gray Code thereis never more than a change of one bit for each adjacent position on thecode disc 14.

The converter I7 converts from Gray to binary code which as will be seenis the code in which character selection is made from the keyboard 19.However, it should be noted there is no necessity that there be exactidentity of the bit positions between a selected character and itsencoded position.

Printing element position decoder 18 is connected to receive the fivemost significant bits from the converter I7. The least significant bitfrom the converter is provided as an input to line positioning control24 where it is converted into a positive or negative drive voltage.

Block 19 represents an electronic keyboard which has a conventionallyarranged type character selection keyboard which is not shown.Depression of a type character selection key causes the keyboard 19 toprovide a discrete binary code indicative of the type charactercommanded to be printed.

The coded output from the electronic keyboard is fed to a positioncommand decoder 20 where it is converted into an analog voltage.

Print element position decoder 18 and position command decoder 20 areconventional digital to analog converters wherein each binary code inputis converted to a discrete current output level.

As aforesaid, the electrical form of the Gray Code is converted to thebinary code in converter 17 and then converted into an analog signalcurrent I: where is! encoder. For example, for lower case 2 may be:

Binary Weight 2 2 2' 2' 2' Position Code 0 l 0 l 0 Signal Current ma) 08 0 2 0 Since a position weighing factor of 0.1 ma per position is used,the position of z given by l .0 ma is positions in a negative directionfrom a given reference point. In practice a fixed current of +0.5 ma isadded to give, in the present instance, -0.6 ma as the position of 2:.

Since there are 22 type characters in a row of the printing element 11and each has two encoded positions, there are 22 discrete analogcurrents.

The only difference between the two binary codes representing oneposition is in the least significant bit. This bit is used in the finepositioning mode as will be discussed further on in this specification.

The keyboard 19 provides a discrete seven bit binary code for eachcharacter selection key depressed. However, two of these bits are usedfor row selection i.e., tilting of the printing element ll via asolenoid arrangement. Five bit words are more than sufficient to encodeeach of the characters on the keyboard since each five bit code can beused once for each of the four rows i.e., for each of the four rows thesame five bit word may stand for a different type character.

For keyboard selection of the character z the output from the keyboard19 is converted into an analog current lit in the position commanddecoder 20 e.g., in a manner similar to:

Binary Weight 2 2' 2' 2 2" Position Code 0 0 l 1 0 Signal Current (ma) 00 .4 .2 0

The current [it generated would then be 0.6 ma.

The analog signals from printing element position decoder 18 andposition command decoder 20 are fed into a summing amplifier 21 to forman error voltage proportional to the absolute value of llH-leproportional in magnitude and direction to the distance the printingelement 11 is away from the commanded position. This error voltage whichhas a scale factor of :l volt/position is used to drive the servo motor12 to position the printing element in the coarse mode.

Thus, an error signal voltage having a magnitude and sense dependent onthe character key depressed and the position of the printing element llis provided on the output terminal of the summing amplifier 21. Thiserror signal is fed as the driving voltage to the motor l2 via normallyconductive transistor 23, servo amplifier 25 and gate 27. The motor [2turns the printing element 11 to the commanded position. As the motorturns the error signal is diminished until it becomes zero. At this timethe sector containing the selected character is positioned but themidpoint of the sector where the type character is actually located maynot be accurately positioned. This is accomplished in the finepositioning mode to be described subsequently.

Block 27 represents a gate which may be an electronic switch such as atransistor. it receives an enabling signal from the keyboard each time akey is depressed and serves as an enabling gate such that the selectionmotor 12 is operative for a sufficient amount of time to complete theselection process.

The servo amplifier 25 is conventional. It runs saturated current modei.e., fully "on" regardless of the magnitude of the input. lts output ispositive or negative depending on the sign of the input. Thus, the motor12 always receives full current i.e., the energizing current doesntdiminish as the error signal voltage is reduced. Thus, the motoracceleration is maximized. The high angular velocity as well as the lackof resolution of the position encoding of only 44 positions in the 360predicate the use of rate damping of the motor 12. A rate sensingnetwork 26 which may be a C.E.M.F. Sensor measures applied motor voltageand actual motor current is used to compute the counter EMF of the motor[2, this value being proportional to rate of speed of the motor 12. Thisvoltage is fed into the servo amplifier 25 along with the error voltagei.e., each voltage is fed to a common junction P on the input terminalof the servo amplifier 25 where they are summed.

During character selection as the printing element 1! approaches itscommanded position, the error voltage A approaches zero while the rateseming voltage B increases due to motor acceleration. As best seen inthe timing diagram of FIG. 6, motor 1! begins braking when the ratesensing voltage relative to the error voltage becomes greater. FIG. 6shows this in curve C which corresponds to the voltage at point C inFIG. 2. As seen in FIG. 6 the voltage at point P passes through zerowhen the rate sensing voltage B becomes greater in magnitude than theerror voltage A. A negative current applied to the motor 12 then causesit to slow down. During this time the voltage B is diminishing becausethe motor 12 is decelerating. Error voltage A continues to be reduceduntil voltage C again passes through zero. At this point motor 12 isroughly positioned although voltage C may oscillate temporarily aboutpoint zero. The error voltage A is then zero.

It should be remembered that depending on the character selecting keydepressed, the error voltage could have been negative. The rate sensingvoltage would then have been positive since the motor 12 would have beendriven in the opposite direction. The effect is still the same i.e., themotor is driven to roughly position the printing element 11.

The rate sensing circuit is conventional having parameters chosen sothat its voltage is opposite in sign to the error voltage while keepingthe same order of magnitude as the error voltage. When the motor isroughly positioned, the fine positioning control 24 described more fullyhereinbelow in reference to FIG. 4 becomes operative.

Fine positioning control 24 includes a circuit which provides a voltagein response to a signal from zero voltage detector 22 to turn oil thetransistor 23 when the error voltage has been reduced to zero.

The fine positioning control 24 also provides positive or negativedriving voltages to the motor 12. The same signal from the zero voltagedetector 22 which initiates the turn off voltage to the transistor 23also enables logic circuitry within the fine positioning control torespond to the least significant bits of the position codes. The codeconverter 17 provides an input to the line positioning control 24 forthe purpose of feeding to it the least significant or homing bits. Thesehoming bits are used to position the center of the character position orsector precisely. The homing bits are the least significant bitsobtained after conversion from the Gray to binary code. The high and lowbits from the code converter 17 cause the motor 12 to oscillate andtherefore the midpoint of the sector of the selected character on theprinting element 11 to oscillate about the commanded position. Thisoccurs because the code disc 14 is oscillating about a correspondingslot, no-slot point. The oscillations become smaller in duration untilthe printing element 11 zeros in on and is hovering closely about itscommanded position at which time it is locked into position e.g., by adetent arrangement (not shown). Somewhat in advance of this the printingelement has been propelled on its forward path to impact.

The physical environment of a single element printer and particularly asingle element typewriter imposes physical restrictions on the servomotor used. For example, the motor must be relatively short along itslongitudinal dimension to prevent interference with the interworkings ofthe printer or typewriter. It must have high torque e.g., 20-25 ozlin.Typical dc motors with this high torque are 2 to 3 inches long since thepermanent magnets which they require must be long to provide the torque.Therefore, a motor built by lnland Motor Company and designated by NT1368 which is only 0.697 inch in length is used. However to provide hightorque output its permanent magnet must be magnetized to its maximum. Inthis state the permanent magnet is subject to demagnetization if itsoverdriven with a consequent reduction of torque. Since in the presentinvention it is necessary to operate the motor at absolute maximum, itis necessary at all times to provide it with full driving current. Toavoid demagnetization, however, the driving current may never exceed therated limit.

As previously pointed out the polarity of the current applied to thetorque motor is abruptly changed to start braking it when the errorvoltage and the rate sensing voltage reach a predetermined relationship.The back EMF of the motor then becomes additive to the applied voltageand the motor would receive a driving current above the rated limit andbe so damaged that the field magnet would have to be replaced orremagnetized.

Therefore, in order to use the selected motor without damaging it isnecessary to provide a power amplifier having symmetrical currentlimiting which will always provide peak current. Such a power amplifiermust provide a constant output current unaffected by load resistance.Thus, variations in the resistance of the servo motor armature windingwhether due to variations in temperature or some other cause will notaffect the output current. For the given supply voltage of 5 volts theload resistance may vary from 8 ohms all the way down to ohms withoutchanging the output current.

FIG. illustrates the power amplifier 27 of the present invention whichaccomplishes the foregoing. A transistor T1 of the NPN type has itsemitter connected to ground through a resistance Rx. Resistance R: isthe only resistance which ever needs to be adjusted or changed tocontrol the reference currents as will be described further on in thisspecification.

The base of transistor T1 is connected to a reference voltage sourcee.g., 5v and its collector is connected to the base of PNP transistorT2. Resistance R1 and R2 are connected respectively to the collector oftransistor T1 and the emitter of transistor T2 and in common to apositive voltage source 3+ through a third resistance R3. The collectorof transistor T2 is connected to a negative voltage power source B-through resistance R4 and RS. In a practical embodiment the positive andnegative voltage sources were 25 volts in magnitude.

The transistor T1 and T2 generate equal reference voltages in resistanceR3 and R5 controllable from a common point i.e., resistance Rx andneither of which is affected by line variation.

The foregoing is accomplished in the following manner:

The transistor T1 generates a constant current through resistance R1which provides a constant voltage across R1. This voltage causes thetransistor T2 to generate a constant current in its collector circuit.The constant currents in resistances R1 and R2 are summed in resistanceR3 providing a constant reference voltage across resistance R3. Inasmuchas transistor T2 is driven via transistor T1 its current is proportionalto the current in transistor T1. Therefore, the value of resistance R5may be chosen so that the voltage thereacross is equal to the voltageacross resistance R3.

Thus, if the value of resistance of R: is changed, it will equallyaffect the voltages across resistance R3 and R5. Therefore, the voltagesacross each of resistances R3 and R5 are constant reference sourcescontrollable from a common point with neither voltage able to beaffected by the external reference voltage variation without the otherbeing equally affected. This eliminates need for external control of thevoltages B+ and B.

The transistor T3, T4 and T5 together form a conventional three stageamplifier for amplifying positive going signals applied to the base ofthe transistor T3 via input terminal 28. The voltage on the inputterminal 28 is limited by means of Zener diodes Z1 and 22 fromoverdriving the amplifier.

Transistors T7, T8 and T9 likewise form a three stage amplifier foramplifying negative going signals applied to the input terminal 28.

The biasing parameters of the amplifier comprising the transistors T3,T4 and T5 are chosen such that any positive voltage (limited to 5 voltsas aforesaid) applied to the input tenninal 28 fully saturates theamplifier and causes full load current to flow in the load resistance R6which is connected between the positive power source 8+ and the emitterof the transistor T5.

ln a similar manner full current flow is provided in the resistance R7connected between the negative power source B- and the emitter of thetransistor '19 when a negative signal is applied to the input terminal28.

The collectors of the transistors T5 and T9 are commonly connected tooutput terminal 29 from which the motor 12 is energized. As should beclear the current through load resistanoes R6 and R7 is positive ornegative depending on the sign of the input voltage. Since either one orthe other transistor T5 or T9 is conductive (in a practical embodimentthis is :13 amperes), the motor 12 receives full driving or brakingcurrent during coarse positioning.

As previously pointed out while it is necessary to drive the motor 12 atits rated maximum, any overdrive will cause its permanent magnet todemagnetize and it will lose its torque. Therefore, the power amplifier27 limits the current in the load resistances R6 and R7 which otherwisemight increase beyond the prescribed limit due to the back EMF of themotor 12 and. its additive eflect when the motor 12 is abruptly brakedas explained heretofore.

The constant reference voltages of resistances R3 and R5 provide thecurrent limiting feature as explained hereinbelow.

Transistors T10 and T11 which are of the PNP type form a difierentialamplifier in the positive signal section of the amplifier andtransistors T12 and T13 which are of the NPN type form a differentialamplifier in the negative signal section of the amplifier.

The emitters of the transistors T10 and T11 are connected to thepositive power source B+ through resistance R8 and the emitters of thetransistors T12 and T13 are connected to the negative power source B-via resistance R9.

The collectors of transistors T11 and T13 are connected to groundthrough resistances R10 and R11, respectively. The collector oftransistor T10 is connected to the emitter of the transistor T3 via aresistance R12. The collector of the transistor T12 is connected to theemitter of the transistor T7 via a resistance R13.

The voltage across the resistance R3 biases the transistor T1 I normallyon. When, however, the current in the resistance R6 reaches the limit,the voltage across resistance R6 becomes equal to the voltage across R3causing the transistors T10 and T11 to begin to conduct current equally.This new current in the transistor T10 flows through the resistances R12and R14 to ground causing the transistor T3 to become back biased andaccept less input which in turn limits the drive also to the transistorsT4 and T5. Accordingly, the load current in resistance R6 is limited andthe motor 12 is not overdriven.

In a similar manner for the negative section of the power amplifiercurrent is made to flow equally in the transistors T 12 and T13 when thevoltage (now negative) in R7 equals the reference voltage in resistanceR5. Then current flows to ground through resistance R13 and R15 to makethe transistors T7, T8 and T9 less conductive to limit the current inresistance R7 to its maximum negative value to prevent overdrive of themotor 12.

The resistances R16 through R21 are the ordinary resistances found inconventional amplifiers of this type. A capacitor (not shown) may beconnected between the collectors of the transistors T3 and T4 andtransistors T7 and T8 if necessary to prevent oscillation.

The output current from the power amplifier is, in addition to not beinga function of load resistance, also not a function of supply voltages asshould now be clear.

The power amplifier 27, therefore, insures maximum drive for the motorat all times but limits current symmetrically to prevent damage. Thereference voltages are adjustable from the common point Rx which may bedone at the factory.

FIG. 4 discloses the detail of the logic of fine positioning control 24which centers the printing element 11 about the midpoint of the 16sector. In the binary code (i.e., after Gray to binary decoding) thispoint may be considered as located where the transition from high to lowor low to high takes place in the least significant bit.

Logic elements L1, L2, L3, L7 and L8 are inverter circuits which converthigh inputs to lows and low inputs to highs.

Logic elements L4, L5, L6, L9 and L10 are NAND circuits which provide alow output when both inputs are high and a high output when both inputsare unalike, or both low.

Logic element Lx is an exclusive OR circuit i.e., it has a high outputwhen one or the other inputs are high. When both inputs are alike, theoutput of OR circuit Lx is low.

The output terminal A of the zero voltage detector 22 is connected tothe input terminal of inverter circuit L1 and to one of the inputterminals of the NAND circuits L9 and L10. The zero voltage detectorprovides a high on its output terminal A when coarse positioning iscomplete i.e., when the error voltage has been reduced to zero. Beforecoarse positioning has been attained, the zero voltage detector 22provides a low on its output terminal.

The low on the output terminal A causes the output of inverter circuitLl to be released (i.e., able to go high providing the input to L3 islow).

The OR circuit Lx has input terminals B and C. The input terminal B isconnected to the keyboard 19 which provides a high (1 when the characterto be selected is upper case and a low when the character to be selectedis lower case.

The second input terminal C of the 0R circuit Lx is coupled to the Grayencoded output of the optical pick-up 16 which provides a high on theinput terminal C when the position encoded is in the upper case sectionof i.e., the first I80 of the printing element I I. A low is provided onthe input terminal C when the position encoded is in the lower casesection or the second I80 of the printing element. Therefore, when theservo motor 12 is selecting within the upper or lower case area of theprinting element alone, the inputs to the OR circuit Lx are both highsor both lows causing a low at its output. This low provided to theinverted L3 along with the low provided at the input of inverter Llforces a low from the output of the inverter circuit L2.

The output terminal of the inverter L2 is connected to the base of thetransistor OI whose collector in turn is connected to the base of theswitching transistor 23. The low on the output of the inverter L2maintains transistors Q1 and 23 conductive. This causes the errorvoltage output from the current summing amplifier 21 to be available atthe servo amplifier 25 through the transistor 23.

Once coarse positioning is attained the transistor 23 is turned offsince the output terminal A goes high causing a low at the output of theinverter L1 and therefore a high at the output of the inverter L2. Thishigh causes transistors 01 and 23 to become non-conductive, thusdisconnecting the current summing amplifier 21 from the servo amplifier25 to prevent backloading of the fine positioning signal during finepositioning by the current summing amplifier 2i.

Simultaneous with this event the fine positioning mode is initiated. Thehigh on the output terminal A is provided as one of the inputs to eachof NAND circuits L9 and L10. The input terminal F provides the leastsignificant bit (the homing bit) from the converter 17 as the secondinput to the NAND circuit L9 via inverter circuit L8 and the secondinput to the NAND circuit L10.

The output terminal of the NAND circuit L10 is connected to the base oftransistor 03. The collector of the transistor 03 is connected to apositive source (+lv) via a load resistance while its emitter isgrounded. The collector of the transistor O3 is connected to the servoamplifier 25 via resistance R25.

The output terminal of the NAND circuit L9 is connected to the base of atransistor 04. The collector of transistor 04 is connected to the baseof a transistor Q5. The collector of the transistor ()5 is connected toa negative voltage source (-1 5v) through a voltage divider. Thejunction of the voltage divider is connected to the servo amplifier 25via resistance R26.

Since the outputs of NAND circuits L4, L5, L9 and L10 are high duringcoarse positioning, the transistors Q3, Q4 and OS are in the conductivestate during this time so that no positioning signal is availablethrough resistances R25 and R26. When transistors 03 or 04 and 05 areturned off, a positive or negative positioning signal is made availablethrough resistances R25 or R26, respectively. This may occur in twosituations, i.e., in the fine positioning mode or when an overshoot ofthe motor occurs as will be more fully explained hereinbelow.

As aforesaid after coarse positioning has been completed, i.e., theprinting element 1 1, motor 12 and code disc I have come within 18 ofthe selected position, the signal on output terminal A goes high l).

The high on output terminal A is one input to each of NAND circuit L9and L10. Thus, when a high appears on the other input temiinal of theNAN D circuit L10, its output goes low. This turns off transistor Q3causing a positive signal to be supplied to the servo amplifier 25.Altemately, when the NAND circuit L9 receives a high on its other inputterminal via inverter circuit L8, its output goes low. This turns offthe transistors Q4 and 05 thus making available a negative signal to theservo amplifier 25 via the resistance R26.

It can now readily be seen that the signals from resistances R25 and R26supply directional fine positioning information to the motor 12.

In the fine positioning mode the least significant bit of the encodedinformation from converter 17 is made available on the terminal F. Thisbit alternates between high (I) and low (0) as the code disc 14 variesin position about midpoint of the selected [6 sector. In this way thepower amplifier 27 may be fed bidirectional control information. Thus,when the least significant bit is a high, transistor 03 is turned offand a positive signal is supplied to drive the motor 12. When the leastsignificant bit is a low, the output of inverter L9 is low andtransistor Q5 goes off causing a negative signal to be supplied to themotor 12. Thus, the motor oscillates about this high low transitionpoint in the code (the center of the 16 sector) until it is finepositioned and homing is complete.

As previously pointed out the foregoing logic circuitry may be used tocompensate for overshoot of the servo system. Overshoot is caused by thediscontinuity on the code disc 14 (Le, the 36070 transition point on thecode disc 14) defined where code position 22 is contiguous with codeposition 1. As the code disc passes this position the voltage fromdecoder 18 would abruptly drop or rise depending on the direction ofrotation.

Code positions 1 and 22 may be visualized by considering the printingelement 11 which is divided into 22 equal character positions. The codedisc which is attached to and moves with the drive shafi of the printingelement 1] is, therefore, also divided into 22 positions. At the 360/0point on the code disc 14 is the juncture of positions 1 and 22. Thismay be physically located by observing at what point on the code disc 14there is an abrupt change of current from decoder 18.

If the character selected is e.g., at position 2 on the printingelement, counterclockwise of position 2 the error is positive, atposition 2 it would be zero and clockwise of position 2 the errorvoltage would be negative.

If the character selected is at position 1 on the printing element asdefined by the discontinuous point on the code disc, clockwise andcounterclockwise motion from that position results in a positive errorsignal. Thus, a counterclockwise direction is commanded regardless ofthe actual direction of error.

As a result of this, serious positioning error could occur whenselection of the character at position I or 22 is desired. For example,if the code disc 14 and printing element 11 are at position 10 and thecharacter in position 1 is selected at the keyboard, momentum may carrythe printing element 11 and therefore the code disc past the codediscontinuity into position 22. As previously explained this results inthe wrong error voltage polarity. if the servo system were ideal, itwould not overshoot at all but certain system variables e.g., friction,lack of lubrication in the motor bearings, or slight misalignment of thecomponents of the motor shaft cannot be completely controlled etc.,cause it. Therefore, a possibility of overshoot always ertists andincorrect error voltage will be generated when overshoot occurs atpositions 1 and 22.

Therefore, the logic circuitry of FIG. 4 has the additional function ofproviding a voltage step or level at this critical position of overshootso that the error signal as seen by the servo amplifier 25 isbidirectional and independent of the position selected.

NAND circuit L4 has one input from terminal B and the other from ORcircuit Lx, and inverter circuit L7. The output terminal of NAND circuitis connected to the base of transistor 03.

A NAND circuit L6 has its output connected to an inverter circuit L7.NAND circuit L5 has one input connected to terminal C and the otherinput connected to the output of the inverter circuit L7, and OR circuitLr. The output of the NAND circuit L5 is connected to the base of thetransistor 04.

When the input terminals B and C to OR circuit Lx have unlike signalse.g., l, or 0, 1, its output will be high, if the input to inverter L7is low. The signals will be unalike whenever the selected character isupper case and the encoded position is lower case or vice versa. At thistime NAND circuits L4 and L each have a high on one of their inputterminals.

Now if there is a high on terminal B, the output of the NAN D circuit L4will go low. Transistor Q3 will cease conducting causing a positiveinput signal to be supplied the servo amplifier 25.

Transistor Q4 and Q5 cease conducting, to apply a negative signal to theservo amplifier 25 if terminal C (instead of terminal B) has a high.This is so because the NAND circuit L5 will then have two highs andprovide a low on the base of the transistor 04.

Thus, without added logic the servo amplifier would receivebidirectional signals whenever the selected position and the encodedposition are in different cases using the "homing bit in an identicalway as described with respect to the fine positioning mode.

Naturally, overshoot protection is needed only when the selectedpositions are l and 22 and because it is only at this position thatdanger of overshooting past the discontinuity of the code is present.

Therefore, the NAND circuits L4 and L5 inputs from the OR circuit Lx areinhibited for all except positions I and 22.

This is accomplished by the NAND circuit L6 having input terminals D andE. The output of the NAND circuit L6 is connected as the input to theinverter circuit L7 whose output terminal is connected to the outputterminal of the OR circuit Lx.

When the inputs on the terminals D and E are both highs, it can be seenthat, the NAND circuits L4 and L5 are enabled, (if B and C aredissimilar) and transistor 03 or transistors 04 and 05 may be turnedoff. The inputs on the tenninals are high only when overshoot may occuri.e., selection to positions 1 and 22.

Thus, even if overshoot occurs, the motor 12 is driven in the correctdirection.

When this overshoot protection is put into operation, it is necessary toeliminate the still present error signal from being supplied to theservo amplifier 25 since it is the signal which at that positioncontains the incorrect directional information.

Since the output of the OR circuit L1: is high (as previously explained)when an overshoot is present, the input of inverter circuit L3 goes highcausing the output of inverter circuit L2 to go high (the output of theinverter LI having no effect). When the output of inverter circuit L2goes high, the transistors Q1 and 23 turn ofi disconnecting the summingamplifier 21 from the servo amplifier 25. The signals on D and E areobtained from the Gray to binary code converter 17 (the fourth bitindicative of the discontinuity) and from optical pickup 16respectively.

Both signals on D and E are the unconverted" Gray coded signals. Thesignal on E is available directly at the optical pickup amplifieroutput, but the signal on terminal D can not be used in its condition asit is at the optical pickup amplifier output, but must be inverted. TheGray to binary converter 17 includes inverters in its logic, and thesignal on terminal D is taken away from the Gray to binary converter 17after it is inverted, but not yet converted.

I claim:

1. A single element printer control for positioning a printing elementin response to a coded command,

position encoding means providing two discrete binary codes for eachposition, each of said two codes difl'ering from each other in the leastsignificant bit.

selection means providing a discrete binary code for each characterselected for printing,

means converting the coded output from each of said position encodingmeans and said selection means into analog l first summing meansconnected to said converting means combining said analog voltages andproviding an error voltage representative of the diflerence in magnitudeand direction of the commanded and actual positions of the printingelement,

servo motor means connected to said summing means coarsely positioningthe printing element to the commanded position in response to said errorvoltage,

fine positioning means connected to said position encoding means andresponsive to changes in the least significant bits of the binary codeof printing element position to finely position the printing element tothe commanded position,

said fine pofltioning control means including detector means connectedto said summing means responsive to zero error voltage to actuate saidfine positioning control means,

said fine positioning also comprising,

transistor switch means normally connecting said first summing means tosaid servo motor means,

first logic means connected between said transistor switch means andsaid detector means for opening said transistor switch means when saiderror signal becomes zero,

second logic means connected between said detector means and said servomotor means and responsive to zero error voltage for providingdirectional driving voltage to said servo motor means in response to theleast significant bits from said position encoding means,

said second logic means further including a positive and negative sourceof voltage,

first transistor means connecting said positive source of voltage tosaid servo motor means in response to zero error voltage and a highleast significant bit,

second transistor means connecting said negative source of voltage tosaid servo motor means in response to zero error voltage and a low leastsignificant bit.

2. A single element printer control system for positioning a printingelement in response to a coded command,

position encoding means providing two discrete binary codes for eachposition, each of said two codes differing from each other in the leastsignificant bit.

selection means providing a discrete binary code for each characterselected for printing,

means converting the coded output from each of said position encodingmeans and said selection means into analog voltages,

first summing means connected to said converting means combining saidanalog voltages and providing an error voltage representative of thedifierence in magnitude and direction of the commanded and actualpositions of the printing element,

servo motor means connected to said summing means coarsely positioningthe printing element to the commanded position in response to said errorvoltage,

fine positioning means connected to said position encoding means andresponsive to changes in the least significant bits of the binary codeof printing element position to finely position the printing element tothe commanded position,

said fine positioning control means including detector means connectedto said summing means responsive to zero error voltage to actuate saidfine positioning control means,

said servo motor means comprising current limiting means for limitingdriving current to said servo motor means regardless of the additiveeffects of back EMF when polarity of the driving current is abruptlyreversed.

3. A single element printer control system for positioning a printingelement in response to a coded command,

position encoding means providing two discrete binary codes for eachposition, each of said two codes differing from each other in the leastsignificant bit,

selection means providing a discrete binary code for each characterselected for printing,

means convening the coded output from each of said position encodingmeans and said selection means into analog voltages,

first summing means connected to said converting means combining saidanalog voltages and providing an error voltage representative of thedifference in magnitude and direction of the commanded and actualpositions of the printing element,

servo motor means connected to said summing means coarsely positioningthe printing element to the commanded position in response to said errorvoltage,

fine positioning means connected to said position encoding means andresponsive to changes in the least significant bits of the binary codeof printing element position to finely position the printing element tothe commanded position,

said fine positioning control means including detector means connectedto said summing means responsive to zero error voltage to actuate saidfine positioning control means, rate damping means including sensormeans connected to said servo motor means providing a signalrepresentative of the instantaneous speed of said servo motor means,

second summing means connected to said first summing means and saidsensor means for combining said error voltage and said speed signal,

amplifier means connected between said second summing means and saidservo motor means applying a driving current to said servo motor meanshaving a polarity dependent on the ratio of error voltages to speedsignal,

said servo motor means comprising current limiting means for limitingdriving current to said servo motor means regardless of the additiveeffects of back EMG when polarity of the driving current is abruptlyreversed.

4. A single element printer control system in accordance with claim 3wherein said fine positioning means comprises,

transistor switch means normally connecting said first summing means tosaid amplifier, first logic means connected between said transistorswitch means and said detector means for opening said transistor switchmeans when said error signal becomes zero,

second logic means connected between said detector means and saidamplifier and responsive to zero error voltage for providing directionaldriving voltage to said amplifier in response to the least significantbits from said position encoding means.

5. A single element printer control system in accordance with claim 4wherein said second logic means includes a positive and negative sourceof voltage,

first transistor means connecting said positive source of voltage tosaid amplifier in response to zero error voltage and a high leastsignificant bit,

second transistor means connecting said negative source of voltage tosaid amplifier in response to zero error voltage and a low leastsignificant bit.

6. A single element printer control system in accordance with claim 5wherein said second logic means includes means for causing said firstand second transistor means to connect said positive or negative voltagesource to said amplifier in response to high and low least significantbits, respectively when predetermined characters are selected regardlessof the presence of error voltage.

7. A single element printer control system according to claim 2 whereinsaid current limiting means comprises a symetrical load current limitingpower amplifier having,

first and second multistage amplifier sections,

said first section comprising a PNP transistor in the output stagehaving its emitter connected to a positive power source through a loadresistance,

said second section comprising a NPN transistor in the output stagehaving its emitter connected to a negative power source through a loadresistance,

an output terminal connected in common to the collectors of saidtransistors,

each of said first and second amplifier stages having a transistor inthe input stage having bases connected in common to an input terminal,

load current limiting means for applying a current to the emitter of theinput transistor of the first section when current through the loadresistor of the output transistor reaches a predetermined amount and forapplying a current to the emitter of the input transistor of the secondsection when current through the load resistor of the output transistorof the second sections reaches a predetermined amount,

whereby the currents applied to the input transistors prevent themagnitude of the load currents from exceeding said predetermined amount.

8. A single element printer control system according to claim 2 whereinsaid current limiting means comprises a symetrical load current limitingpower amplifier having,

first and second differential amplifiers,

said first differential amplifier comprising first and second PNPtransistors having their emitters commonly connected to a positive powersource,

said second diflerential amplifier comprising first and second NPNtransistors having their emitters commonly connected to a negative powersource,

first amplifier means for amplifying all positive going signalscomprising at least an input transistor having an emitter connected tothe collector of said first PNP transistor and an output transistorhaving an emitter connected to the base of said first PNP transistor,

second amplifier means for amplifying all negative going signalscomprising at least an input transistor having an emitter connected tothe collector of said first NPN transistor and an output transistorhaving an emitter connected to the base of said first N PN transistor,

the bases of each of said input transistors connected in common to aninput terminal,

the collectors of each of said output transistors being connected incommon to an output terminal,

a load resistor in the emitter circuit of each of said outputtransistors,

a first load resistor connected between a positive source of voltage andthe emitter of one of said output transistors,

a second load resistor connected between a negative source of voltageand the emitter of the other of said output transistors,

means biasing said first and second differential amplifier to permitcurrent flow only in each of said second transistors thereof,

each of said differential amplifiers responsive to the current in eitherof the load resistors attaining a predetermined value to cause therespective first transistors thereof to become conductive,

whereby current is fed back to reduce conduction of the appropriateinput transistor.

9. A single element printer control system according to claim 1 whereinsaid servo motor means comprises,

a DC torque motor having its output shaft connected to the printingelement,

said amplifier means being connected between said second summing meansand said DC torque motor,

said amplifier means including current control means maintaining saiddriving current at near peak value without exceeding a predeterminedvalue even when the polarity of the driving current is abruptly changed.

10. A single element printer control system according to claim 9 whereinsaid current control means comprises,

first and second multistage amplifier sections,

said first section comprising a PNP transistor in the output stagehaving its emitter connected to a positive power source through a loadresistance,

claim 9 wherein said current control means comprises,

said second section comprising a NPN transistor in the output stagehaving its emitter connected to a negative power source through a loadresistance,

an output terminal connected in common to the collectors of saidtransistors and to the energin'ng coil of said DC 5 torque motor,

each of said first and second amplifier stages having a transistor inthe input stage having bases connected in common to an input terminal,

said input terminal connected to said second summing 1 means,

load current limiting means for applying a current to the emitter of theinput transistor of the first section when current through the loadresistor of the output transistor reaches a predetermined amount and forapplying a curl rent to the emitter of the input transistor of thesecond section when current through the load resistor of the outputtransistor of the second sections reaches a predetermined amount,

whereby the currents applied to the input transistors prevent themagnitude of the load currents from exceeding said predetermined amount.

11. A single element printer control system according to first andsecond differential amplifiers,

said first differential amplifier comprising first and second PNPtransistors having their emitters commonly connected to a positive powersource,

said second difi'erential amplifier comprising first and second NPNtransistors having their emitters commonly connected to a negative powersource,

first amplifier means for amplifying all positive going signalscomprising at least an input transistor having an emitter connected tothe collector of said first PNP transistor and an output transistorhaving an emitter connected to the base of said first PNP transistor,

second amplifier means for amplifying all negative going signalscomprising at least an input transistor having an emitter connected tothe collector of said first NPN transistor and an output transistorhaving an emitter connected to the base of said first NPN transistor,

the bases of each of said input transistors connected in common to aninput terminal,

said input terminal connected to said second summing means,

the collectors of each of said output transistors being connected incommon to an output terminal,

said output terminal being connected to the energizing coil of said DCtorque motor,

a load resistor connected between a positive source of voltage and theemitter of one of said output transistors,

a second load resistor connected between a negative source of voltageand the emitter of the other of said output transistors,

means biasing said first and second differential amplifier to permitcurrent flow only in each of said second transistors thereof,

each of said differential amplifiers responsive to the current in eitherof the load resistors attaining a predetermined value to cause therespective first transistors thereof to become conductive,

whereby current is fed back to reduce conduction of the appropriateinput transistor.

I I i i

1. A single element printer control for positioning a printing element in response to a coded command, position encoding means providing two discrete binary codes for each position, each of said two codes differing from each other in the least significant bit. selection means providing a discrete binary code for each character selected for printing, means converting the coded output from each of said position encoding means and said selection means into analog voltages, first summing means connected to said converting means combining said analog voltages and providing an error voltage representative of the difference in magnitude and direction of the commanded and actual positions of the printing element, servo motor means connected to said summing means coarsely positioning the printing element to the commanded position in response to said error voltage, fine positioning means connected to said position encoding means and responsive to changes in the least significant bits of the binary code of printing element position to finely position the printing element to the commanded position, said fine positioning control means including detector means connected to said summing means responsive to zero error voltage to actuate said fine positioning control means, said fine positioning also comprising, transistor switch means normally connecting said first summing means to said servo motor means, first logic means connected between said transistor switch means and said detector means for opening said transistor switch means when said error signal becomes zero, second logic means connected between said detector means and said servo motor means and responsive to zero error voltage for providing directional driving voltage to said servo motor means in response to the least significant bits from said position encoding means, said second logic means further including a positive and negative source of voltage, first transistor means connecting said positive source of voltage to said servo motor means in response to zero error voltage and a high least significant bit, second transistor means connecting said negative source of voltage to said servo motor means in response to zero error voltage and a low least significant bit.
 2. A single element printer control system for positioning a printing element in response to a coded command, position encoding means providing two discrete binary codes for each position, each of said two codes differing from each other in the least significant bit. selection means providing a discrete binary code for each character selected for printing, means converting the coded output from each of said position encoding means and said selection means into analog voltages, first summing means connected to said converting means combining said analog voltages and providing an error voltage representative of the difference in magnitude and direction of the commanded and actual positions of the printing element, servo motor means connected to said summing means coarsely positioning the printing element to the commanded position in response to said error voltage, fine positioning means connected to said position encoding means and responsive to changes in the least significant bits of the binary code of printing element position to finely position the printing element to the commanded position, said fine positioning control means including detector means connected to said Summing means responsive to zero error voltage to actuate said fine positioning control means, said servo motor means comprising current limiting means for limiting driving current to said servo motor means regardless of the additive effects of back EMF when polarity of the driving current is abruptly reversed.
 3. A single element printer control system for positioning a printing element in response to a coded command, position encoding means providing two discrete binary codes for each position, each of said two codes differing from each other in the least significant bit, selection means providing a discrete binary code for each character selected for printing, means converting the coded output from each of said position encoding means and said selection means into analog voltages, first summing means connected to said converting means combining said analog voltages and providing an error voltage representative of the difference in magnitude and direction of the commanded and actual positions of the printing element, servo motor means connected to said summing means coarsely positioning the printing element to the commanded position in response to said error voltage, fine positioning means connected to said position encoding means and responsive to changes in the least significant bits of the binary code of printing element position to finely position the printing element to the commanded position, said fine positioning control means including detector means connected to said summing means responsive to zero error voltage to actuate said fine positioning control means, rate damping means including sensor means connected to said servo motor means providing a signal representative of the instantaneous speed of said servo motor means, second summing means connected to said first summing means and said sensor means for combining said error voltage and said speed signal, amplifier means connected between said second summing means and said servo motor means applying a driving current to said servo motor means having a polarity dependent on the ratio of error voltages to speed signal, said servo motor means comprising current limiting means for limiting driving current to said servo motor means regardless of the additive effects of back EMG when polarity of the driving current is abruptly reversed.
 4. A single element printer control system in accordance with claim 3 wherein said fine positioning means comprises, transistor switch means normally connecting said first summing means to said amplifier, first logic means connected between said transistor switch means and said detector means for opening said transistor switch means when said error signal becomes zero, second logic means connected between said detector means and said amplifier and responsive to zero error voltage for providing directional driving voltage to said amplifier in response to the least significant bits from said position encoding means.
 5. A single element printer control system in accordance with claim 4 wherein said second logic means includes a positive and negative source of voltage, first transistor means connecting said positive source of voltage to said amplifier in response to zero error voltage and a high least significant bit, second transistor means connecting said negative source of voltage to said amplifier in response to zero error voltage and a low least significant bit.
 6. A single element printer control system in accordance with claim 5 wherein said second logic means includes means for causing said first and second transistor means to connect said positive or negative voltage source to said amplifier in response to high and low least significant bits, respectively when predetermined characters are selected regardless of the presence of error voltage.
 7. A single element printer control system according to claim 2 wherein said current limiting means comprises a symetrical load currEnt limiting power amplifier having, first and second multistage amplifier sections, said first section comprising a PNP transistor in the output stage having its emitter connected to a positive power source through a load resistance, said second section comprising a NPN transistor in the output stage having its emitter connected to a negative power source through a load resistance, an output terminal connected in common to the collectors of said transistors, each of said first and second amplifier stages having a transistor in the input stage having bases connected in common to an input terminal, load current limiting means for applying a current to the emitter of the input transistor of the first section when current through the load resistor of the output transistor reaches a predetermined amount and for applying a current to the emitter of the input transistor of the second section when current through the load resistor of the output transistor of the second sections reaches a predetermined amount, whereby the currents applied to the input transistors prevent the magnitude of the load currents from exceeding said predetermined amount.
 8. A single element printer control system according to claim 2 wherein said current limiting means comprises a symetrical load current limiting power amplifier having, first and second differential amplifiers, said first differential amplifier comprising first and second PNP transistors having their emitters commonly connected to a positive power source, said second differential amplifier comprising first and second NPN transistors having their emitters commonly connected to a negative power source, first amplifier means for amplifying all positive going signals comprising at least an input transistor having an emitter connected to the collector of said first PNP transistor and an output transistor having an emitter connected to the base of said first PNP transistor, second amplifier means for amplifying all negative going signals comprising at least an input transistor having an emitter connected to the collector of said first NPN transistor and an output transistor having an emitter connected to the base of said first NPN transistor, the bases of each of said input transistors connected in common to an input terminal, the collectors of each of said output transistors being connected in common to an output terminal, a load resistor in the emitter circuit of each of said output transistors, a first load resistor connected between a positive source of voltage and the emitter of one of said output transistors, a second load resistor connected between a negative source of voltage and the emitter of the other of said output transistors, means biasing said first and second differential amplifier to permit current flow only in each of said second transistors thereof, each of said differential amplifiers responsive to the current in either of the load resistors attaining a predetermined value to cause the respective first transistors thereof to become conductive, whereby current is fed back to reduce conduction of the appropriate input transistor.
 9. A single element printer control system according to claim 1 wherein said servo motor means comprises, a DC torque motor having its output shaft connected to the printing element, said amplifier means being connected between said second summing means and said DC torque motor, said amplifier means including current control means maintaining said driving current at near peak value without exceeding a predetermined value even when the polarity of the driving current is abruptly changed.
 10. A single element printer control system according to claim 9 wherein said current control means comprises, first and second multistage amplifier sections, said first section comprising a PNP transistor in the output stage having its emitter connected to a positive power Source through a load resistance, said second section comprising a NPN transistor in the output stage having its emitter connected to a negative power source through a load resistance, an output terminal connected in common to the collectors of said transistors and to the energizing coil of said DC torque motor, each of said first and second amplifier stages having a transistor in the input stage having bases connected in common to an input terminal, said input terminal connected to said second summing means, load current limiting means for applying a current to the emitter of the input transistor of the first section when current through the load resistor of the output transistor reaches a predetermined amount and for applying a current to the emitter of the input transistor of the second section when current through the load resistor of the output transistor of the second sections reaches a predetermined amount, whereby the currents applied to the input transistors prevent the magnitude of the load currents from exceeding said predetermined amount.
 11. A single element printer control system according to claim 9 wherein said current control means comprises, first and second differential amplifiers, said first differential amplifier comprising first and second PNP transistors having their emitters commonly connected to a positive power source, said second differential amplifier comprising first and second NPN transistors having their emitters commonly connected to a negative power source, first amplifier means for amplifying all positive going signals comprising at least an input transistor having an emitter connected to the collector of said first PNP transistor and an output transistor having an emitter connected to the base of said first PNP transistor, second amplifier means for amplifying all negative going signals comprising at least an input transistor having an emitter connected to the collector of said first NPN transistor and an output transistor having an emitter connected to the base of said first NPN transistor, the bases of each of said input transistors connected in common to an input terminal, said input terminal connected to said second summing means, the collectors of each of said output transistors being connected in common to an output terminal, said output terminal being connected to the energizing coil of said DC torque motor, a load resistor connected between a positive source of voltage and the emitter of one of said output transistors, a second load resistor connected between a negative source of voltage and the emitter of the other of said output transistors, means biasing said first and second differential amplifier to permit current flow only in each of said second transistors thereof, each of said differential amplifiers responsive to the current in either of the load resistors attaining a predetermined value to cause the respective first transistors thereof to become conductive, whereby current is fed back to reduce conduction of the appropriate input transistor. 